Patent GB-680103-A (GB)
Title: Release signal system for telegraph exchange systems
Inventor: OBERMAN ROELOF MAARTEN MARIE
IPC: H04L12/02
Filed: 1947-12-30
Abstract: 680,103. Telegraph exchange systems. STAATSBEDRIJF DER POSTERIJEN TELEGRAPHIE EN TELEFONIE. Dec. 30, 1947 [April 29, 1944], No. 34843/47. Class 40 (iii). In a telegraph exchange system a supervisory relay in an exchange repeating arrangement is energized only after it has received a clearing signal of constant marking current for a predetermined time, the relay then transmitting spacing current to the clearing subscriber so that this subscriber receives a signal consisting of a marking start element, five marking code elements and a spacing stop element. In the continental code employed this signal represents " letters shift " and thus causes a single revolution of the subscriber's teleprinter cam-shaft so that it returns to the correct starting position without printing a letter, and ensures that, both upon disconnecting and re-connecting, the teleprinter does not, owing to its cam-shaft having stopped in an arbitrary position, print a false character. Establishing a connection. The subscriber initiating a call closes a calling switch OT (Fig. 1), to provide a calling loop which operates relay AA to maintain the loop over aa1 and also operates the line relay LR at the exchange (Fig. 2) which starts a line-finder OZ to connect the calling line to free selecting equipment, which may be a register. If the exchange is a manual one then the relay LR merely provides the usual calling indication. When the necessary equipment has been seized a relay KB (not shown) operates over kb1, a cut-off relay COR which at cor1 and cor2 reverses the line polarity. The relay AA releases, the calling loop being maintained through a start relay AB which operates and at ab1, ab2 supplies rectified A.C. to an auxiliary start relay AH which operates to start the subscriber's teleprinter motor MA over ah1 and ah2. At contact ah3 a short-circuit is removed from the receiving magnet BA which is thus only now connected to line, and since the normal rest condition of the line current has now been established false operation of the receiver is prevented. The start relay AB releases at ah4 but the auxiliary start relay AH is maintained operated over contacts ah6, ah5, and ac1, and maintains the calling loop over ah3 and the receive relay BA. The subscriber may now operate this keyboard to select the wanted line, in a known manner. Releasing a connection. Each subscriber has the local equipment shown in Fig. 1, and at the exchange has the repeating equipment shown in Fig. 2. Either subscriber may close a release switch ST to energize a slow-release relay AD which at ad1, ad2 disconnects the receiver magnet BA from the line and connects it instead to the local source of current S4. During the change-over of these contacts the receiver cam-shaft starts to rotate but the local source of current thereafter supplies only marking current, and the signal recorded is therefore " letter-shift " and causes no printing. At ad3 the rectifier S2 is reversed across the line and breaks the battery circuit, causing the transmit relay A of the exchange repeater to which the subscriber is connected to stay in the marking condition. Let us suppose that the clear key was closed by the subscriber not shown so that his repeater transmit relay (not shown) moves its armature b (Fig. 2) on to its marking contact as described above for the relay A. As described in application 34841/47 as placed open to public inspection under Sect. 91 (4), after 110 ms. a condenser C1 charges sufficiently to fire a discharge tube B and so operate a relay LB after a further 10 ms. Contact lb4 therefore sends a stop (spacing) signal to the subscriber shown after six marking elements, causing the receiver cam-shaft to rotate for one revolution. No printing takes place since this signal is " lettersshift ". Both the cam-shafts are now normal and are held' either by local current from the rectifier bridge S4 or by line current. A relay LD is operated at kb4 when a call is set up and is used to seize switching equipment, and the operating circuit of this relay is now switched to a low resistance holding coil at lb2, the operating coil being disconnected at lb3. The battery applied at lb2 releases the cut-off relay COR, reversing the line current at cor1, cor2. This reversal operates release relay AC at the remote subscriber (i.e. the one shown) and also starts the remote cam-shaft which makes one revolution, but without printing, as described above. Relay AD is operated over ac2 and at ad1, ad2 switches the receiver over to a local current source which locks it. Relay AH releases at ac1, and at ah3 releases in turn relay AC. AD releases over ac2 with a delay sufficient to ensure that the receiver motor has first stopped. When the line current is reversed at cor1 and cor2 the line relay A in the equipment shown in Fig. 2 changes over to its marking contact and its contact a (not shown) initiates in the equipment of the subscriber who closed the clear key a chain of action similar to that of the contact b. Both sets of equipment are thus returned to rest, and the release of the relays LD releases any equipment seized when the call was set up. In a modification, Fig. 3 (not shown), the functions of relay AD in Fig. 1 are shared with a further relay AE. The slow release relay AD switches the receiver to a local current source of line polarity so that the receiver remains locked and operates relay AE which in turn reverses the connection of the line to the rectifier S2. The cut-off relay COR therefore releases and reverses the line current, operating relay AC and so releasing the auxiliary start relay AH which in turn stops the motor. If the calling key OT is now closed the exchange will reverse the line current and operate the start relay AB as before, additional contacts of which connect the receiver to line although relay AD has not yet released. Signalling can therefore recommence after a disconnection without waiting for the slow release relay AD to release.